The EN 300 421 standard of the ETSI (European Telecommunications Standards Institute) relates to DVB services over transparent satellite communication systems. The purpose of this standard is to furnish direct-to-user services known as DVB-S (digital video broadcasting via satellite), through an integrated receiver/decoder device that is located in the user's home. Its versatility in multiplexing permits the use of a transmission capacity encompassing a variety of television (TV) service configurations, including sound and data services. All the components of said services are time division multiplexed (TDM) on a single carrier. The most detailed description of this standard can be found in the ETSI publication, EN 300 421 V1.1.2 (1997–98) entitled: “Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for 11/12 GHz satellite services” whose content is included in this description by reference.
Moreover, the ETSI standard known as DVB-RCS001 makes reference to interaction channels on a transparent satellite distribution system. The purpose of this standard is to furnish basic specifications for the provision of interaction channels for interactive networks based on geo-stationary (GEO) satellites that incorporate fixed return channel satellite terminals (RCST). The service is also known as DVB-RCS (digital video broadcasting—return channel satellite). This standard facilitates the use of RCSTs for domestic installations both individual and collective types. It likewise supports the connection of said terminals with data networks at home, and is applicable to all the frequency bands allocated to GEO satellite services. The most detailed description of this standard can be found in the ETSI publication, TM2267r3DVB-RCS001rev12 (11 Feb. 2000) entitled: “Digital Video Broadcasting (DVB); Interaction Channel for Satellite Distribution Systems”, whose content is included in this description by reference.
These two standards are of the mono-spot type; that is, the satellite defines a single zone as coverage zone. The mono-spot systems therefore present the drawback that, by having limited coverage zones, they are not suitable for more extensive areas on a worldwide scale. The services related with each of said standards are presently employed in a mutually independent form.
The steadily growing user demand for interactive services makes it necessary for satellite communication systems to support broadcasting with return channel to the end users when the latter may be scattered over entirely different and dislike regions of the world, and thereby facilitate better access and faster interconnection between them. This in turn make it necessary the provision of systems capable of broadcasting on networks that support multimedia having a multispot communication characteristic.
The service that DVB-S provides, although offering the possibility of direct communication to the user's home, has the drawback of not having foreseen the possibility of including a return channel in order that the user may communicate with the multimedia service provider. Also, the services furnished by DVB-RCS include said return channel, but do not provide the possibility of direct communication with the user's home for broadcast applications.
In the light of the foregoing, it has become necessary to facilitate an integrated multispot satellite communication system on a multimedia broadcasting network capable of supporting digital video broadcasting (DVB) applications in order to facilitate multimedia services directly to the user's home and permitting at the same time that said user can establish communication with the multimedia service provider over a return channel.
One solution to this problem, proposed by this same applicant, has been to offer the users a multimedia broadcast service such that the user may communicate with the multimedia service provider via a return channel to the satellite, all of this in a substantially economical manner.
Said solution proposed the combined use of the standardised DVB-S and DVB-RCS services, thereby obtaining a single, regenerative and multispot satellite system permitting the use of standard stations both on the sending and on the receiving side. Through said solution, both the end user and the multimedia service provider employ a return channel in accordance with the DVB-RCS standard via an uplink channel to the satellite.
On board the satellite the regenerative payload performs the multiplexing of the information coming from various sources into a data stream suitable for being received by a user who has available any standard integrated receiver/decoder equipment.
Moreover, according to the ETSI standard DVB-RCS001 (draft EN 301 790 V1.1.1.) relative to the interaction channels in satellite systems, the synchronisation of the bursts transmitted by the return channel satellite terminal (RCST) is carried out by receiving information on a network clock reference (NCR) inside the standard packets termed MPEG2-TS (Motion Picture Expert Group 2-Transport Stream), sent by a network control centre in DVB-S format. The RCST reconstructs the original 27 MHz reference of the network control centre, permitting it to transmit the return information in burst mode in an allocated time slot.
As has already been mentioned, the mutually independent and transparent use of the two broadcasting and interaction services performed in a conventional manner makes it unnecessary to have synchronization of the bursts in transmission of the programs offered to the users with the user-to-provider return service, thus the RCST could strictly employ the DVB-RCS standard.
However, on combining the two services, DVB-S and DVB-RCS, that is, when using an integrated broadcasting and interaction system as has been mentioned above, it is assumed that the RCSTs used by the broadcast provider shall be the same as those used by the users. It has therefore been chosen to carry out a burst synchronization that is common both for the interactive services and for broadcasting, and consequently permits the use of the same type of RCST on the service provider side and on the user side.
In the present state of the art, the rules for encoding moving pictures together with the associated audio signal within MPEG2-TS packets, as well as the synchronisation of the encoding system, are defined in the ISO/IEC 13818-1 standard. Said rules are based on receiving Program Clock Reference (PCR) fields. The PCR is the time reference in the transport stream from which the timing for decoding is obtained. The decoders reconstruct the program clock starting with these values and their respective arrival times.
According to the timing model in the recommendations of ISO/IEC 13818-1, it is assumed that all digitised pictures, as well as the audio samples that are applied to the encoder, appear once at the output of the decoder, after a fixed, end-to-end delay. Thus, the sample rates, both of video frames and of audio samples, in the decoder are exactly the same as in the encoder.
In the event of a re-multiplexing operation between the encoding and decoding functions, correction of the PCRs becomes necessary.
Said correction is conventionally achieved by the addition of a correction factor to the PCR according to the following formula:ΔPCR=delcurr−delconstwhere delcurr is the current delay experienced by the PCR and delconst is a constant value that is used for all the PCRs of the program in question.
This solution is only valid if the MPEG2 packets are transmitted in continuous mode, that is, the information inserted by the encoder is kept valid at the output from the modulation stage. However, in the event of the MPEG2 packets being transmitted in burst mode—as has been mentioned above—the solution of correction by the scheme explained above will not achieve the desired result since it does not take into account the compression effect produced in TDMA (time division multiple access) mode. This would be the case in the integrated satellite systems in interactive networks for performing multimedia broadcasting with return channel in which the RCSTs transmit MPEG2 transport streams using the MF-TDMA access mode defined in the DVB-RCS standard.
Consequently, it is necessary to employ a new method of correcting the Program Clock Reference (PCR) that is applicable to broadcasting systems that transmit MPEG2 transport streams according to the access mode defined in the DVB-RCS standard.